1. Field of the Invention
This invention relates to alkyl t-alkyl ethers and more particularly relates to a process for separating such ethers from alkanols.
2. Description of the Prior Art
As shown in Netherlands published application No. 7,807,035 (Snam Progetti); U.S. Pat. Nos. 3,940,450 (Lee) and 4,148,695 (Lee et al.); John C. Davis et al., "MTBE bandwagon," CHEMICAL ENGINEERING, May 21, 1979, pp. 91-93; and Stephen C. Stinson, "New plants, processes set for octane booster," CHEMICAL & ENGINEERING NEWS, June 25, 1979, pp. 35 and 36, it is known that alkyl t-alkyl ethers can be prepared by reacting an isoolefin with an alkanol in the presence of an ion exchange resin. Such reactions have general interest, although the greatest current interest is in the reaction of isobutylene with methanol to form methyl t-butyl ether. Methyl t-butyl ether has utility as an additive for improving the octane rating of gasoline.
In the reactions of isoolefins with alkanols, it is preferred to use an excess of alkanol in order to minimize the formation of by-products. However, the use of an excess of alkanol results in contaminating the ether product with a difficultly removable impurity. The alkanol forms an azeotrope with the ether and therefore cannot be separated therefrom by simple atmospheric distillation.
It is frequently desirable to separate the alkanol from the ether. It is also desirable to separate the alkanol from any unreacted hydrocarbons in the crude reaction product, since alkanols are poisons for alkylation catalysts and, if not removed, would make the unreacted hydrocarbons unsuitable for use in alkylation units. Thus, since atmospheric distillation canot be used, it is conventional to remove the alkanol by pressure distillation. This method of removing the alkanol is costly in terms of energy requirements and capital investment.
An economical and efficient process for separating alkanols from alkyl t-alkyl ethers and unreacted hydrocarbons is disclosed in copending application Ser. No. 69,210, filed Aug. 23, 1979, in the names of Anderson O. Dotson, Jr., and Francis T. Wadsworth. In the process of that application, solid or aqueous calcium chloride is intimately admixed with an ether-alkanol mixture to form a calcium chloride-alkanol complex which is easily separable from the ether. The application teaches that the aqueous solutions are generally more efficient than solid calcium chloride and are particularly suitable for use in processes wherein the alkanol is to be recovered and recycled to a reactor.
Copending application Ser. No. 142,272, filed Apr. 21, 1980, in the names of Anderson O. Dotson, Jr., and Francis T. Wadsworth, now U.S. Pat. No. 4,262,149, teaches an improvement over the preferred process of Ser. No. 69,210, wherein an aqueous calcium chloride-alkanol complex is contacted with a C.sub.4 or C.sub.5 hydrocarbon feed comprising an isoolefin at a temperature of about 80.degree.-120.degree. C. and a pressure of about 20-180 psig to separate the components of the complex and permit their reuse.
Although the processes of the aforementioned copending applications are economical and efficient, they have one disadvantage. The use of aqueous calcium chloride to complex the alkanol creates corrosion problems.